This invention relates to the fluidization of fine powders.
One particular example of a manufacturing process that may involve fluidization of fine powders is the provision of valve-metal coated powder for compaction to form capacitor anodes of electrolytic capacitors.
Thus for instance particles of an inert material such as alumina may be coated with a layer of tantalum to provide a coated powder which can be compacted to form an anode having similar electrical properties to that of an anode made of particles of comparable size made entirely of tantalum. One advantage of using the coated power is in this instance the cost saving resulting from the use of less tantalum. In the case of coating the particles with aluminum an advantage of using aluminum coated powder over particles composed entirely of aluminum is that the coated powder is easier to compact into a form suitable for a capacitor anode because the hard cores of the particles are useful in transmitting the compaction forces through the body of material being compacted and are also useful in limiting the extent of the compaction so that body is still porous after compaction. A convenient way of coating the particles is by a chemical vapor reaction process performed in a fluidized powder bed.
In order to make a coated powder compacted anode that is superior or at least competitive with anodes made of powder that is made exclusively of valve-metal it is generally desirable to use coated powder that has a particular size that is comparable with that of the valve-metal powder. In recent years there has been a move towards using finer powders so that now powders with a particle size in the range 1 to 5 microns are used. Whereas it has been found in a series of tests relatively easy to fluidize alumina powders down to a particle size of about 13 microns, the fluidization of 9 micron powder could not be reliably achieved in a conventional spouted bed fluidization chamber, and fluidization of 3 micron powder was not achieved at all.